The Bike That Phill Built: Rear Derailleur

The rear dearailleur is the component that shifts the chain from cog to cog on the rear sprockets. It is a fairly complex piece of equipment as the sprockets sizes vary quite a bit and as such the derailleur also must adjust chain tension to cater for slack created when in the smallest gear.

For small item it sure is complicated!

So not only do I have to choose a derailleur that fits the cassette but also that fits my crankset size as well. This means there were a few things I had to be wary of when choosing this item - the maximum and minimum sprocket sizes on the cassette, the difference between the chainring sizes on the front sprockets, and the total capacity - which is the difference between the maximum and minimum chainring sizes on the crankset added to the difference between the largest and smallest cog in the cassette. The total capacity is a measurement of the range in which the pulley of the rear derailleur can comfortbaly operate.

Here I am just adjusting releiving some (chain) tension

Other things I had to look out for were long or short cage - short cages work faster and more smoothly but usually do not fit large cassettes, and the shifting action (top or low) - this has to do with easier down shifting and really only affects technical moutain bikers.

My rear derailleur was an item I bought super cheap from Chain Reaction Cycles so I splashed out a bit and got the item I really wanted - the Shimano Deore XT Shadow M772.

While installation instructions were included I'm pretty confident if I just rest it right here on the wheel everything will just hold itself together.

Model Number	RD-M772-SGS
Series	DEORE XT
Shifting Action	Top Normal SHADOW
Mount Type	Drop-out
Maximum Sprocket	34T
Minimum Sprocket	11T
Front Difference	22T maximum
Total Capacity	45T
Guide Pulley	sealed ceramic bushing
Tension Pulley	sealed bearing

Its a long cage to fit my 9-speed cassette with the top normal shifting action. My cassette's smallest sprocket is 11 teeth and largest is 32, my crankset chainring difference (Front Difference) is 44-22=22, and my Total Capacity = (44-22)+(32-11) = 43.

Its getting longer and longer to set this up!

Shimano Deore XT Shadow M772 Rear Derailleur = $108.00 (157.09RRP - 30% off)
My only regret is buying to soon - Chain Reaction are now selling them for $78!

Total
Frame Set $1099+
Wheel Set $ 495
Handlebars $135
Headset $ 77.50
Cassette $ 89.90
Crankset $198.02
Rear Derailleur $108

$2202.42